Radically polymerizable photo-curable ink jet ink composition

ABSTRACT

A radically polymerizable photo-curable ink jet ink composition contains a radically polymerizable compound, a white pigment, a polymer dispersant, and at least one saturated aliphatic carboxylic acid.

BACKGROUND

1. Technical Field

The present invention relates to a radically polymerizable photo-curableink jet ink composition.

2. Related Art

Ink jet recording ink compositions containing a white pigment are known.For example, JP-A-2007-99833 discloses a white ink jet ink compositionmainly containing a white pigment, a polymer dispersant, a photo-curablecompound, and a photopolymerization initiator for enhancing thedispersion stability of the ink composition to minimize the settling ofthe pigment in order to prevent the ink from clogging nozzles. In thisink, the polymer dispersant has a sulfonate group.

However, in the known ink compositions containing a white pigment,viscous sediment is deposited during long-time storage even if the inkcomposition contains a dispersant. The deposited sediment is difficultto redisperse sufficiently even by agitating the ink composition. Theknown white pigment-containing ink compositions are inferior inredispersing sediment deposited during long-time storage to restore itto a former dispersion, that is, inferior in redispersibility afterlong-time storage.

SUMMARY

An advantage of some aspects of the invention is that it provides aradically polymerizable photo-curable ink jet ink composition in whichsediment deposited during long-time storage can be easily redispersedeven though it contains a white pigment.

The present inventors have found through their intensive research thatthe above problem can be solved by adding a saturated aliphaticcarboxylic acid to a radically polymerizable photo-curable ink jet inkcomposition containing a white pigment.

According to an aspect of the invention, the following radicallypolymerizable photo-curable ink jet ink composition is provided. Theradically polymerizable photo-curable ink jet ink composition contains aradically polymerizable compound, a white pigment, a polymer dispersant,and at least one saturated aliphatic carboxylic acid.

The saturated aliphatic carboxylic acid may be at least one compoundselected from the group consisting of saturated aliphatic monocarboxylicacids and saturated aliphatic dicarboxylic acids.

The saturated aliphatic carboxylic acid may have 2 to 15 carbon atoms inthe portion other than the carboxyl group.

The saturated aliphatic carboxylic acid content may be in the range of0.05% to 0.2% by mass relative to the total mass of the ink composition.

The radically polymerizable photo-curable ink jet ink composition mayfurther contain a photopolymerization initiator.

The white pigment may contain metal oxide particles.

The saturated aliphatic carboxylic acid may be at least one compoundselected from the group consisting of the compounds expressed by generalformulas (1) and (2):COOH—(CH₂)_(n)—COOH  (1); andCH₃—(CH₂)_(n-1)—COOH  (2),

wherein n represents an integer in the range of 2 to 15.

The n of the general formulas may represent an integer of 3 to 8.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will now be described. However, theinvention is not limited to the disclosed embodiments, and variousmodifications may be made without departing from the scope and spirit ofthe invention.

The radically polymerizable photo-curable ink jet ink composition of anembodiment of the invention contains a radically polymerizable compound,a white pigment, a polymer dispersant, and at least one saturatedaliphatic carboxylic acid.

Radically Polymerizable Compound

The ink composition of the present embodiment contains a radicallypolymerizable compound. Radically polymerizable compounds are liable toproduce a viscous sediment in an ink composition during long-timestorage. The concept of the present disclosure is effective in such acase.

Radically polymerizable compounds that can be used in the presentembodiment include monofunctional, bifunctional, trifunctional,tetrafunctional, pentafunctional and hexafunctional (meth)acrylates.Although radically polymerizable compounds will be further described indetail, the radically polymerizable compound used in the presentembodiment is not limited to the following. In the description,(meth)acrylate refers to an acrylate and the corresponding methacrylate.

Exemplary monofunctional (meth)acrylates include, but are not limitedto, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, tert-octyl(meth)acrylate, isoamyl (meth)acrylate, decyl (meth)acrylate, isodecyl(meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate,cyclohexyl (meth)acrylate, 4-n-butylcyclohexyl (meth)acrylate, bornyl(meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate,2-ethylhexyl diglycol (meth)acrylate, butoxyethyl (meth)acrylate,2-chloroethyl (meth)acrylate, 4-bromobutyl (meth)acrylate, cyanoethyl(meth)acrylate, benzyl (meth)acrylate, butoxymethyl (meth)acrylate,3-methoxybutyl (meth)acrylate, alkoxymethyl (meth)acrylate, alkoxyethyl(meth)acrylate, 2-(2-methoxyethoxy)ethyl (meth)acrylate,2-(2-butoxyethoxy)ethyl (meth)acrylate, 2,2,2-tetrafluoroethyl(meth)acrylate, 1H,1H,2H,2H-perfluorodecyl (meth)acrylate, 4-butylphenyl(meth)acrylate, phenyl (meth)acrylate, 2,4,5-tetramethylphenyl(meth)acrylate, 4-chlorophenyl (meth)acrylate, phenoxymethyl(meth)acrylate, phenoxyethyl (meth)acrylate, glycidyl (meth)acrylate,glycidyloxybutyl (meth)acrylate, glycidyloxyethyl (meth)acrylate,glycidyloxypropyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate,hydroxyalkyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, dimethylaminoethyl (meth)acrylate,diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate,diethylaminopropyl (meth)acrylate, trimethoxysilylpropyl (meth)acrylate,trimethylsilylpropyl (meth)acrylate, polyethylene oxide monomethyl ether(meth)acrylate, oligoethylene oxide monomethyl ether (meth)acrylate,polyethylene oxide (meth)acrylate, oligoethylene oxide (meth)acrylate,oligoethylene oxide monoalkyl ether (meth)acrylate, polyethylene oxidemonoalkyl ether (meth)acrylate, dipropylene glycol (meth)acrylate,polypropylene oxide monoalkyl ether (meth)acrylate, oligopropylene oxidemonoalkyl ether (meth)acrylate, 2-methacryloyloxyethysuccinic acid,2-methacryloyloxyhexahydrophthalic acid,2-methacryloyloxyethyl-2-hydroxypropyl phthalate, butoxy diethyleneglycol (meth)acrylate, trifluoroethyl (meth)acrylate,perfluorooctylethyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, ethylene oxide-modified phenol (meth)acrylate, ethyleneoxide-modified cresol (meth)acrylate, ethylene oxide-modifiednonylphenol (meth)acrylate, polyethylene oxide-modified nonylphenol(meth)acrylate, ethylene oxide-modified 2-ethylhexyl (meth)acrylate,Carbitol (meth)acrylate, oligoester (meth)acrylate, epoxy(meth)acrylate, urethane (meth)acrylate, methyl (meth)acrylate, n-butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate,allyl (meth)acrylate, glycidyl (meth)acrylate, benzyl (meth)acrylate,and dimethylaminomethyl (meth)acrylate.

Exemplary bifunctional (meth)acrylates include, but are not limited to,1,6-hexanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate,neopentyl glycol di(meth)acrylate, 2,4-dimethyl-1,5-pentanedioldi(meth)acrylate, butylethylpropanediol di(meth)acrylate, ethoxylatedcyclohexanemethanol di(meth)acrylate, polyethylene glycoldi(meth)acrylate, oligoethylene glycol di(meth)acrylate, ethylene glycoldi(meth)acrylate, 2-ethyl-2-butyl-butanediol di(meth)acrylate,hydroxypivalic acid neopentyl glycol di(meth)acrylate, ethyleneoxide-modified bisphenol A di(meth)acrylate, bisphenol F polyethoxydi(meth)acrylate, polypropylene glycol di(meth)acrylate, oligopropyleneglycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,2-ethyl-2-butylpropanediol di(meth)acrylate, 1,9-nonanedi(meth)acrylate, propoxylated-ethoxylated bisphenol A di(meth)acrylate,tricyclodecane di(meth)acrylate, diethylene glycol di(meth)acrylate,triethylene glycol di(meth)acrylate, tetraethylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, andpolypropylene glycol di(meth)acrylate.

Exemplary trifunctional (meth)acrylates include, but are not limited to,trimethylolpropane tri(meth)acrylate, trimethylolethanetri(meth)acrylate, alkylene oxide-modified trimethylolpropanetri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritoltri(meth)acrylate, trimethylolpropane tri((meth)acryloyloxypropyl)ether,isocyanuric acid alkylene oxide-modified tri(meth)acrylate, propionicacid dipentaerythritol tri(meth)acrylate, tri((meth)acryloyloxyethyl)isocyanurate, hydroxypivalic aldehyde-modified dimethylolpropanetri(meth)acrylate, sorbitol tri(meth)acrylate, propoxylatedtrimethylolpropane tri(meth)acrylate, ethoxylated glycerin triacrylate,trimethylolethane tri(meth)acrylate, and trimethylolpropanetri(meth)acrylate.

Exemplary tetrafunctional (meth)acrylates include, but are not limitedto, pentaerythritol tetra(meth)acrylate, sorbitol tetra(meth)acrylate,ditrimethylolpropane tetra(meth)acrylate, propionic aciddipentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritoltetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, andtetramethylolmethane tetra(meth)acrylate.

Exemplary pentafunctional (meth)acrylates include, but are not limitedto, sorbitol penta(meth)acrylate and dipentaerythritolpenta(meth)acrylate.

Exemplary hexafunctional (meth)acrylates include, but are not limitedto, dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate,alkylene oxide-modified phosphazene hexa(meth)acrylate, andcaprolactone-modified dipentaerythritol hexa(meth)acrylate.

Other radically polymerizable compounds may be used without particularlimitation, including unsaturated carboxylic acids and salts thereof,such as acrylic acid, methacrylic acid, itaconic acid, crotonic acid,isocrotonic acid, and maleic acid, esters, urethane, amides, anhydrides,acrylonitrile, styrene, unsaturated polyesters, unsaturated polyethers,unsaturated polyamides, and unsaturated urethanes. More specifically,examples of these radically polymerizable compounds include, but are notlimited to, acrylic acid derivatives, such asbis(4-acryloxypolyethoxyphenyl)propane and diacetone acrylamide;methacrylic acid derivatives, such as2,2-bis(4-methacryloxypolyethoxyphenyl)propane; allyl compoundderivatives, such as allyl glycidyl ether, diallyl phthalate, andtriallyl trimellitate; commercially available compounds disclosed in“Crosslinking Agent Handbook” (in Japanese, S. Yamashita, 1981,Taiseisha), “UV/EB Curing Handbook (Material Edition)” (in Japanese, K.Kato, 1985, Kobunshi Kankokai), “Applications and Markets of UV/EBCuring Technology” (in Japanese, Edited by RadTech Japan, p. 79, 1989,CMC Publishing), or “Polyester Resin Handbook” (in Japanese, E.Takiyama, 1988, the Nikkan Kogyo Shinbun), and radically polymerizableor crosslinkable monomers, oligomers and polymers known in the art.

From the viewpoint of easy curing, acrylates are advantageously used.

The above-cited radially polymerizable compounds may be used singly orin combination.

The content of the radically polymerizable compound is preferably 40% to80% by mass, more preferably 60% to 80% by mass, still more preferably65% to 75% by mass, relative to the total mass of the ink composition.The radically polymerizable compound with such a content can be easilydissolved in the ink composition, and the use of such an amount ofradically polymerizable compound allows the ink composition to maintaina low viscosity and can enhance the curability and storage stability ofthe ink composition.

White Pigment

The ink composition of the present embodiment contains a white pigment.White pigments generally have higher specific gravity than otherpigments and are liable to settle accordingly. The concept of thepresent disclosure is effective in the case of using a white pigment.The white pigment may be, but is not limited to, a commerciallyavailable inorganic or organic white pigment or hollow particles. Thewhite pigment used in the present embodiment may have a specific gravityin the range of 3.8 to 4.1. The present disclosure is particularlyeffective in such a case.

Examples of the inorganic white pigment include, but are not limited to,titanium oxide, calcium carbonate, barium sulfate, silica fine powderand other forms of silica, calcium silicate, alumina, zinc oxide, ceriumoxide, talc, and clay.

Preferably, the white pigment contains metal oxide fine particles. Metaloxide fine particles generally have higher refractive indices and areaccordingly likely to exhibit higher whiteness than other materials.Examples of the metal oxide fine particles include, but are not limitedto, particles of alumina, titanium oxide, zinc oxide and cerium oxide.In particular, titanium oxide fine particles can exhibit satisfactoryhiding power and coloring power as a pigment. Titanium oxide fineparticles generally have a lower specific gravity and a higherrefractive index than other inorganic white pigments and are chemicallyand physically stable. Also, titanium oxide is generally resistant toacids and alkalis and other conditions.

Examples of the organic white pigment include, but are not limited to,organic salts disclosed in JP-A-11-129613 and alkylene-bis-melaminederivatives disclosed in JP-A-11-140365 and JP-A-2001-234093.Commercially available organic white pigments include Shigenox OWP,Shigenox OWPL, Shigenox FWP, Shigenox FWG, Shigenox UL, and Shigenox U(each produced by Hakko Chemical).

The hollow particles may be, but are not limited to, thermoplasticparticles substantially made of organic polymer, disclosed in U.S. Pat.No. 4,089,800.

Preferably, the white pigment is treated with silica, alumina, zincoxide, zirconia, or an organic compound. The white pigment treated withthese materials is, in general, dispersible, lipophilic and hydrophilicand resistant to weather.

It is known that white pigments treated with alumina, silica, zirconiaor the like have OH groups at the surfaces thereof, and thatalumina-treated white pigment surfaces are rather neutral,silica-treated surfaces are acid, and zirconia-treated surfaces arebasic.

Preferably, the white pigment particles have an average particle size of100 nm or more, more preferably 200 nm or more. The upper limit of theaverage particle size of the white pigment particles is preferably 10μm, more preferably 1 μm, and still more preferably 300 nm. The averageparticle size can be controlled by selecting an appropriate pigment,dispersant and dispersion medium, and setting appropriate dispersion andfiltering conditions. By controlling the particle size in the aboverange, the resulting ink composition can be stably ejected. In addition,when the white pigment has such an average particle size, the inkcomposition is likely to exhibit satisfactory whiteness and hidingpower. The average particle size can be measured with a particle sizedistribution meter (for example, UPA-EX 150 manufactured by Nikkiso).

The white pigment content of the ink composition is preferably 5% to 50%by mass, more preferably 5% to 30% by mass, still more preferably 5% to20% by mass, relative to the total mass of the ink composition. The useof the white pigment with such a content allows the ink composition tomaintain a low viscosity and exhibit good storage stability and highredispersibility after long-time storage.

In the preparation of the ink composition, before being mixed with othermaterials, the white pigment may be dispersed in a dispersion medium andthus used in the form of dispersion. The white pigment can be dispersedwith any dispersion device. Examples of the dispersion device include,but are not limited to, a ball mill, a sand mill, an attritor, a rollmill, a jet mill, a homogenizer, a paint shaker, a kneader, an agitator,a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearlmill, and a wet jet mill.

The dispersion medium for the white pigment dispersion may be, but isnot limited to, a radically polymerizable compound. A low-viscosityradically polymerizable compound is preferred. In general, the use ofsuch a dispersion medium allows the white pigment to dispersesatisfactorily and enables the ink composition to be easily handled.

Polymer Dispersant

The ink composition of the present embodiment contains a polymerdispersant. The polymer dispersant interacts with the functional groupof the white pigment to attach to the surfaces of the white pigmentparticles. Consequently, the white pigment is dispersed in the inkcomposition by the steric hindrance or electrical repulsion of theattached polymer dispersant. Therefore, the polymer dispersant can beappropriately selected according to the charge and other conditions atthe surfaces of the white pigment particles. The polymer dispersantpreferably has at least either an acid value or an amine value, and morepreferably has both at a comparable level. The use of such a polymerdispersant enables alumina-treated white pigment, silica-treated whitepigment and zirconia-treated white pigment to disperse moresatisfactorily. The term polymer dispersant used herein refers to adispersant of a polymer having a repeating unit, and particularly refersto a resin dispersant. Preferably, the polymer dispersant may have amolecular weight of, but not limited to, 1,000 to 50,000.

Examples of the polymer dispersant include, but are not limited to,vinyl resins, such as polyvinyl alcohols and polyvinyl pyrrolidone;acrylic resins, such as polyacrylic acid, acrylic acid-acrylonitrilecopolymer, potassium acrylate-acrylonitrile copolymer, vinylacetate-acrylic ester copolymer, and acrylic acid-acrylic estercopolymer; styrene-acrylic resins, such as styrene-acrylic acidcopolymer, styrene-methacrylic acid copolymer, styrene-methacrylicacid-acrylic ester copolymer, styrene-α-methylstyrene-acrylic acidcopolymer, and styrene-α-methylstyrene-acrylic acid-acrylic estercopolymer; styrene-maleic acid copolymer; styrene-maleic anhydridecopolymer; vinyl naphthalene-acrylic acid copolymer; vinylnaphthalene-maleic acid copolymer; vinyl acetate-based copolymers, suchas vinyl acetate-ethylene copolymer, vinyl acetate-vinyl ethylene fattyacid copolymer, vinyl acetate-maleic acid ester copolymer, vinylacetate-crotonic acid copolymer, and vinyl acetate-acrylic acidcopolymer; and salts of these polymers. Among these, preferred arecopolymers of a monomer having a hydrophobic group and a monomer havinga hydrophilic group, and polymers whose molecules have both ahydrophobic group and a hydrophilic group. The above polymer dispersantsmay be used singly or in combination.

The polymer dispersant is added preferably in a proportion of 1 to 10parts by mass, more preferably 2 to 7 parts by mass, relative to themass (100 parts by mass) of the white pigment. The addition of thepolymer dispersant in such a proportion allows the white pigment toadsorb the polymer dispersant and disperse satisfactorily, and canminimize the increase in the viscosity of the ink composition caused bythe presence of polymer compounds. In addition, the polymer dispersantis likely to act as a surfactant to reduce the surface tension of theink composition and thus can minimize the occurrence of ink jet ejectionfailure.

Saturated Aliphatic Carboxylic Acid

The ink composition of the present embodiment contains one or moresaturated aliphatic carboxylic acids. The white pigment is dispersed inthe ink composition by the steric hindrance or electrical repulsion ofthe polymer dispersant attached to the surfaces thereof. However, if aknown ink composition containing a polymer dispersant together with awhite pigment is allowed to stand for a long time such as a year ormore, viscous sediment is deposited. The viscous sediment is difficultto redisperse even by agitation.

The reason can be as follows. The polymer dispersant attached to thesurfaces of the white pigment particles will probably separate from thepigment particles as time passes. Once the polymer dispersant hasseparated, the white pigment particles, which have been dispersed by thesteric hindrance and electrical repulsion of the polymer dispersant,aggregate together. The aggregated white pigment settles to form aviscous sediment that is difficult to redisperse and restore to a formerdispersion even by agitation. The reason is not limited to this andother factors may be involved in the occurrence of the viscous sediment.

On the other hand, in the ink composition of the present embodiment,which contains a saturated aliphatic carboxylic acid together with awhite pigment, the polarized saturated aliphatic carboxylic acidattaches to the surfaces of the white pigment particles. Consequently,the pigment particles electrically repel from each other or thesaturated aliphatic carboxylic acid attracts the separated polymerdispersant, and thus the white pigment is stably dispersed. Thesaturated aliphatic carboxylic acid in the ink composition may preventthe polymer dispersant from separating from the pigment particles, andthe white pigment thus can be stably dispersed in the ink composition.

Saturated aliphatic carboxylic acids are carboxylic acids having nounsaturated carbon-carbon bonds and containing no aromatic rings. Thealiphatic skeleton of the saturated aliphatic carboxylic acid may havesubstituents. Examples of the saturated aliphatic carboxylic acidinclude, but are not limited to, succinic acid, glutaric acid, adipicacid, pimelic acid, suberic acid, azelaic acid, sebacic acid, propionicacid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylicacid, pelargonic acid, capric acid, lauric acid, myristic acid, palmiticacid, margaric acid, and citric acid. The aliphatic skeleton may belinear or branched, and is preferably linear. A linear aliphaticskeleton can enable the sediment deposited during long-time storage toredisperse more effectively.

Preferably, the above-mentioned one or more saturated aliphaticcarboxylic acids include at least either a saturated aliphaticmonocarboxylic acid or a saturated aliphatic dicarboxylic acid, and morepreferably include a saturated aliphatic dicarboxylic acid. Thesesaturated aliphatic carboxylic acids can enable the sediment depositedduring long-time storage to redisperse more effectively.

Preferably, the saturated aliphatic carboxylic acid has 2 to 15 carbonatoms, more preferably 3 to 8 carbon atoms, in the portion other thanthe carboxyl group. Such a saturated aliphatic carboxylic acid canenable the sediment deposited during long-time storage to redispersemore effectively.

Preferably, the saturated aliphatic carboxylic acid content of the inkcomposition is 0.05% to 0.25% by mass, more preferably 0.09% to 0.20% bymass, and still more preferably 0.09% to 0.15% by mass, relative to thetotal mass of the ink composition. The use of the saturated aliphaticcarboxylic acid with such a content can enable the sediment depositedduring long-time storage to redisperse more effectively.

Preferably, the saturated aliphatic carboxylic acid contains at leastone compound selected from the group consisting of the compoundsexpressed by the following general formula (1) or (2):COOH—(CH₂)_(n)—COOH  (1)CH₃—(CH₂)_(n-1)—COOH  (2)

wherein n represents an integer in the range of 2 to 15. Such asaturated aliphatic carboxylic acid can enable the sediment depositedduring long-time storage to redisperse more effectively.

The n of the general formulas is preferably an integer of 2 to 15, morepreferably 3 to 8. Such a saturated aliphatic carboxylic acid can enablethe sediment deposited during long-time storage to redisperse moreeffectively.

The acid dissociation constant pKa1 of the saturated aliphaticcarboxylic acid is preferably 4.0 or more, more preferably 4.0 to 4.5,still more preferably 4.1 to 4.5. If the saturated aliphatic carboxylicacid is divalent or more polyvalent, the second acid dissociationconstant pKa2 is preferably 5.0 or more. The acid dissociation constantpKa can be measured by, for example, titration, absorptiometry, orcapillary electrophoresis. Table 1 shows the acid dissociation constantspKa and redispersing effect of exemplary saturated aliphatic carboxylicacids.

TABLE 1 pKa1 pKa2 pKa3 Redispersion Palmitic acid 4.64 Good Adipic acid4.26 5.03 Excellent Succinic acid 4 5.24 Good Citric acid 2.87 4.35 5.69FairPhotopolymerization Initiator

Preferably, the ink composition contains a polymerization initiator. Anycompound may be used as the polymerization initiator without particularlimitation as long as it can cure the radically polymerizable compound,and a known photo-radical polymerization initiator can be used.

Examples of the photo-radial polymerization initiator include, but arenot limited to, aromatic ketones, acylphosphine oxides, thioxanthonecompounds, aromatic onium salts, organic peroxides, thio compounds (suchas thiophenyl group-containing compounds), α-aminoalkylphenonecompounds, hexaaryl biimidazole compounds, ketoxime ester compounds,borates, azinium compounds, metallocene compounds, active estercompounds, compounds having a carbon-halogen bond, and alkylaminecompounds.

Among these compounds, acylphosphine oxides and thioxanthone compoundsare preferred. These photo-radical polymerization initiators generallyfacilitate the curing of the ink composition with a UV light emittingdevice, thus enhancing the curability of the ink composition.

Examples of the acylphosphine oxides include, but are not limited to,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide,(2,4,6-trimethylbenzoyl)diphenylphosphine oxide, andbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

Examples of the thioxanthone compounds include, but are not limited to,thioxanthone, diethylthioxanthone, isopropylthioxanthone, andchlorothioxanthone. These may be used singly or in combination.Preferred diethylthioxanthone may be, but is not limited to,2,4-diethylthioxanthone; preferred isopropylthioxanthone may be, but isnot limited to, 2-isopropylthioxanthone; and preferredchlorothioxanthone may be, but is not limited to, 2-chlorothioxanthone.The ink composition containing such a thioxanthone compound as thephotopolymerization initiator can exhibit high curability, storagestability, and ejection stability. Preferably, the thioxanthone compoundcontains diethylthioxanthone. Diethylthioxanthone can more efficientlyproduce an active species that can initiate radical polymerization withUV light in a wide range of frequency.

Other photo-radical polymerization initiators may be used withoutparticular limitation. Examples of such a photo-radical polymerizationinitiator include acetophenone, acetophenone benzylketal,1-hydroxycyclohexylphenyl ketone, 2,2-dimethoxy-2-phenylacetophenone,xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone,triphenylamine, carbazole, 3-methylacetophenone, 4-chlorobenzophenone,4,4′-dimethoxybenzophenone, 4,4′-diaminobenzophenone, Michler's ketone,benzoin propyl ether, benzoin ethyl ether, benzyldimethylketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one,2-hydroxy-2-methyl-1-phenylpropane-1-one, and2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one.

Other Additives

The ink composition may further contain other additives or constituents.For example, the ink composition may contain known additives, such as,but not limited to, a slipping agent (surfactant), a polymerizationpromoter, a penetration enhancer, a wetting agent (moisturizing agent),a fixing agent, a fungicide, a preservative, an antioxidant, anultraviolet absorbent, a chelating agent, a pH adjuster, and athickener.

Preparation of the Radically Polymerizable Photo-Curable Ink Jet InkComposition

The ink composition can be prepared in any known process withoutparticular limitation. For example, the ink composition can be preparedby mixing the above-described constituents to disperse in each otherwith agitation. Alternatively, a pigment dispersion containing a whitepigment may be prepared in advance, and then other constituents aremixed to the pigment dispersion.

Ink Container

The ink composition of the present embodiment may be provided in theform of an ink container containing the ink composition. The inkcontainer is prepared by placing the ink composition in a container soas to be suitable for transportation. The container portion of the inkcontainer is preferably a bag (ink bag) whose portion in contact withthe ink composition is made of a film, from the viewpoint of reducingthe weight. The film may be made of a resin such as polyethyleneterephthalate, polyethylene, polypropylene, polyvinyl chloride, orethylene-vinyl alcohol copolymer, and ethylene-vinyl alcohol copolymeris suitable in view of low oxygen permeation and high durability. Thefilm may include a resin layer and an inorganic coating made of, forexample, alumina. The film preferably has a thickness of 0.05 to 0.3 mm.

EXAMPLES

The invention will be further described in detail with reference toExamples and Comparative Examples. However, the invention is not limitedto the following Examples.

Materials of Ink Compositions

The following materials were mainly used in the ink compositions of theExamples and Comparative Examples. Radically Polymerizable Compounds:

Monofunctional monomer V#192 (phenoxyethyl acrylate, produced by OsakaOrganic Chemical Industry)

Bifunctional monomer VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, producedby Nippon Shokubai)

Trifunctional monomer SR444 (pentaerythritol triacrylate, produced bySartomer)

Photopolymerization Initiators:

Irgacure 819 (produced by BASF)

Speedcure TPO (produced by Lambson)

Sensitizers:

Speedcure DET (produced by Lambson)

Hostalux KCB (produced by Clariant)

Polymerization Inhibitor:

MEHQ (p-methoxyphenol)

Surfactant:

BYK 3500 (produced by BYK)

White Pigment:

Titanium oxide surface-treated with alumina, CR-50 (produced by IshiharaSangyo Kaisha)

Polymer Dispersant:

BYK 180 (produced by BYK)

Saturated Aliphatic Carboxylic Acid:

Succinic acid

Adipic acid

Lauric acid

Palmitic acid

2-Naphthoic acid

Citric acid

Diethyl adipate

Examples 1 to 9, Comparative Examples 1 to 3

Preparation of Ink Compositions and Ink Bags

Materials were mixed in the proportions (percent by mass) shown in Table2 and sufficiently agitated to yield the ink compositions of Examplesand Comparative Examples. To a 30 cm×15 cm rectangular bag whose foursides except the ink inlet were thermally sealed, 600 mL of any of theink compositions was introduced through the ink inlet, and the ink inletwas sealed in a state where air did not remain in the bag. The bag wasmade of an ethylene-vinyl alcohol copolymer film having a thickness of0.1 mm.

Evaluation

Redispersibility after Long-Time Storage

Ink bags containing respective ink compositions were allowed to stand atroom temperature (25° C.) with the rectangular face of the ink bag in ahorizontal position for a year. After the one year storage, each ink bagwas reciprocally moved 50 times in the direction of the longer sides ofthe rectangular ink bag at a reciprocal shaking width of 5 cm and aspeed of 50 cm/s, with the rectangular face of the ink bag in ahorizontal position. Subsequently, the ink bag was turned upside downand was reciprocally moved 50 times in the same manner with therectangular face of the ink gab in a horizontal position. After the inkcomposition in the ink bag was thus agitated, the ink composition wassampled from the upper side in the ink gab with the ink inlet facing up.Then, the absorbance of the sampled ink composition was measured with aspectrophotometer U-3300 (manufactured by Hitachi) and compared with theabsorbance before the storage. The rate of change in absorbancecalculated from measurements was evaluated according to the followingcriteria, and the results are shown in Table 2. As a larger amount ofsediment settles, the density of the white pigment in the inkcomposition tends to be reduced, and accordingly the absorbance afterstorage is generally low.

Criteria:

AA:  0% to less than 10% A: 10% to less than 20% B: 20% to less than 30%C: 30% or moreSolubility

Ink compositions were prepared as above. The ink compositions werevisibly observed for insoluble residue and evaluated according to thefollowing criteria, and the results are shown in Table 2.

Criteria:

A: Insoluble residue was not observed.

B: Insoluble residue was observed.

Storage Stability

The ink compositions prepared as above were each placed in the bagprepared as above and allowed to stand at 70° C. for 8 days. Before andafter the 8-day storage, the viscosities of the ink compositions weremeasured at 20° C. with a viscometer MCR-300 (manufactured by Physica).The storage stability was evaluated from the rate of change in viscositybefore and after the storage according to the following criteria, andthe results are shown in Table 2.

Criteria:

A: Less than 10% B: 10% or more

TABLE 2 Comparative Comparative Comparative Type Material name Example 1Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8Example 9 Example 1 Example 2 Example 3 Polymerizable MonofunctionalV#192 19.10 19.10 19.10 19.10 19.10 19.10 19.10 19.10 19.10 19.10 19.1019.10 compound monomer Bifunctional VEEA 48.16 48.16 48.19 48.06 48.1648.16 48.16 48.22 48.01 48.26 48.16 48.16 monomer Trifunctional SR4445.55 5.55 5.55 5.55 5.55 5.55 5.55 5.55 5.55 5.55 5.55 5.55 monomerAdditive Photopolymerization Irgacure 819 2.25 2.25 2.25 2.25 2.25 2.252.25 2.25 2.25 2.25 2.25 2.25 initiator Speedcure 4.80 4.80 4.80 4.804.80 4.80 4.80 4.80 4.80 4.80 4.80 4.80 TPO Sensitizer Speedcure 0.500.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 DETX Hostalux KCB0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08Polymerization MEHQ 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.360.36 0.36 inhibitor Surfactant BYK3500 0.20 0.20 0.20 0.20 0.20 0.200.20 0.20 0.20 0.20 0.20 0.20 Pigment Titanium 18.00 18.00 18.00 18.0018.00 18.00 18.00 18.00 18.00 18.00 18.00 18.00 oxide, alumina surface-treated Dispersant BYK180 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.900.90 0.90 0.90 Dispersing assistant Succinic acid 0.10 Adipic acid 0.100.07 0.20 0.04 0.25 Lauric acid 0.10 Palmitic acid 0.10 2-Naphthoic 0.10acid Citric acid 0.10 Diethyl 0.10 adipate Total 100.00 100.00 100.00100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00Redispersibility after long time storage A AA A AA A A B B A C C CSolubility A A A A A A A A B A A A Storage stability A A A A A A A A B AA A

The above results show that the ink composition of the disclosure, whichcontains a specific saturated aliphatic carboxylic acid, has goodredispersibility after long-time storage. In addition, since the inkcomposition has good redispersibility and viscous sediment, even ifformed, can be redispersed by agitation, nozzle clogging or unevennessin the density of the image caused by the settling of the white pigmentcan be minimized. Furthermore, the bag used in the Examples was notbroken or cracked even by agitation, and was thus resistant to agitationfor redispersion.

The entire disclosure of Japanese Patent Application No.: 2013-061556,filed Mar. 25, 2013 is expressly incorporated by reference herein.

What is claimed is:
 1. A radically polymerizable photo-curable ink jetink composition comprising: a radically polymerizable compound, whereinthe radically polymerizable compound content is in the range of 40% to80% by mass relative to the total mass of the ink composition; a whitepigment, wherein the white pigment content is in the range of 5% to 50%by mass relative to the total mass of the ink composition; a polymerdispersant; and at least one saturated aliphatic dicarboxylic acid,wherein the saturated aliphatic dicarboxylic acid content is in therange of 0.05% to 0.25% by mass relative to the total mass of the inkcomposition.
 2. The radically polymerizable photo-polymerizable ink jetink composition according to claim 1, wherein the saturated aliphaticdicarboxylic acid has 2 to 15 carbon atoms in the portion other than thecarboxyl group.
 3. The radically polymerizable photo-curable ink jet inkcompositing according to claim 1, wherein the saturated aliphaticdicarboxylic acid content is in the range of 0.05% to 0.2% by massrelative to the total mass of the ink composition.
 4. The radicallypolymerizable photo-curable ink jet ink composition according to claim1, further comprising a photopolymerization initiator.
 5. The radicallypolymerizable photo-curable ink jet ink composition according to claim1, wherein the white pigment contains metal oxide particles.
 6. Theradically polymerizable photo-curable ink jet ink composition accordingto claim 1, wherein the saturated aliphatic dicarboxylic acid is atleast one compound selected from the general formula (1):COOH—(CH₂)_(n)—COOH  (1); wherein n represents an integer of 2 to
 15. 7.The radically polymerizable photo-curable ink jet ink compositionaccording to claim 6, wherein the n of the general formulas representsan integer of 3 to 8.